Multiple sclerosis (MS) is an inflammatory, demyelinating disease of the central nervous system that is believed to result from erroneous activation of self-reactive T cells specific for myelin antigens. Experimental allergic encephalomyelitis (EAE) is an animal model for MS that is induced by immunization with myelin antigens. In the EAE-susceptible B10.PL strain, research has focused on T cells specific for the immunodominant epitope of myelin basic protein (MBP), AcMBP1-11. Our previous studies showed that the immunodominance of AcMBP1-11 is the product of immune tolerance induced by endogenous expression of MBP. We demonstrated using MBP-deficient mice that MBP121-150 is the most immunogenic region of MBP in the absence of tolerance. MBP121-150-specific T cells are subdominant in wild-type mice because central tolerance mechanisms eliminate most, but not all, of these T cells from the periphery. Using a MBP121-150-specific T cell receptor transgenic mouse model, we showed that the MBP121-150-specific T cells that escape central tolerance are pathogenic because they can be specifically triggered to induce EAE. New data presented here show that transgenic MBP121-150-specific T cells that reside in the periphery are prevented from causing spontaneous disease by regulatory T cells. Adoptive transfer of transgenic, naive MBP121-150-specific T cells into T cell deficient mice results in rapid and severe autoimmune disease. This autoimmunity is completely prevented by introducing CD4+ T cells. Surprisingly, the regulatory T cells do not inhibit expansion of the MBP-specific T cells in vivo after transfer or their migration to the brain. This proposal focuses on defining the phenotype (Aim 1) and mechanisms of action (Aim 2) of the regulatory T cells. We will test the hypothesis that regulatory T cells alter the cytokine milieu at sites of antigen presentation to inhibit activation of Th1 inflammatory T cells. Understanding how regulatory T cells prevent MBP-specific T cells from mediating disease is important for understanding the pathogenesis of MS and may provide insights into new therapeutic strategies.